In 1937, through the use of the electroencephalograph EEG measuring tiny electrical brain impulses, Loomis and his associates discovered that the form of brain-waves change with the onset of sleep. The next leap forward in understanding came when Aserinsky and Kleitman found rapid eye movements REM in 1953. In 1957 the REM were linked with dreaming. This defined sleep into two different observable states, REM sleep, and NREM non rapid eye movement, pronounced non-Rem sleep. Within NREM three different stages have been identified. These are defined by the different EEG patterns of electrical activity in the brain. They are measured by their height amplitude and frequency of up and down movement. There are also electrical changes occurring in the muscles EMG, and in movement of the eyeballs EOG.
